This invention relates generally to mechanical transfer mechanisms for conveying work at a work station from one stage to another, and in particular to an automated work transfer system for conveying a basket carrying work through a row of processing stages in any desired sequence, the stages being of the type involved in the fabrication of microelectronic devices.
By using diffusion techniques it is now possible to fabricate transistors and diodes as well as resistors and capacitors without a single wafer of silicon to create integrated circuits. The manufacture of microelectronic devices involves sequences of photolithography, etching and critical cleaning processes. For the most part, these have heretofore been carried out manually at chemistry work benches. In the usual manufacturing procedure, batches of substrates or silicon wafers are carried in "boats" and transferred from one chemical tank to another by a human operator who adjusts the conditions prevailing in each tank and also determines the periods during which the boat dwells in the respective tanks for treatment therein.
While automated transfer operations are now commonplace in many industrial processes including electroplating, there are a number of practical factors which have militated against the introduction of automation in the processing of microelectronic devices. A major factor is that the value of the microelectronic devices is high relative to the cost of labor involved in making these parts. Thus the introduction of automated work transfer mechanisms to reduce labor costs may not be economically justified, particularly when one takes into account the capital investment dictated by automated equipment.
But other factors now come into play which have little to do with the cost effectiveness of replacing manual operations with automated work transfer mechanisms. As microelectronic devices become increasingly complex and sophisticated, the associated wet chemistry procedures are rendered even more critical. As a consequence, the variable and human errors incidental to manual operation can no longer be tolerated, for repeatability and consistency are now the primary desiderata. Hence these considerations now override the cost effectiveness factor.
Though efforts have been made to automate work transfer operations in wet chemistry processing of microelectronic devices, they have had limited success, largely because of certain environmental problems. Many of the chemical processing tanks discharge corrosive vapors into the region of the work station. In the typical wet chemistry bench, the arrangement is such as to create an air curtain to isolate the corrosive environment at the work station from the external room environment. Since automated work transfer mechanisms of the type heretofore proposed are arranged to operate within the corrosive environment at the work station, special expedients have been required to cope with this environment.
Thus, it has heretofore been the practice to employ corrosion-resistant materials in the manufacture of automated work transfer mechanisms for wet chemistry benches. And to achieve satisfactory and reliable performance, it has also been necessary to purge the housings for the motors which drive the transfer mechanism with nitrogen or external air to exclude corrosive fumes from the motor components.
But such expedients have failed to entirely solve the problem; for while it is possible to protect the transfer mechanism from chemical attack by using corrosion-resistant plastics, one must also isolate the chemical process from contamination by the mechanism, for particulate matter from the mechanism and lubricants used therein may fall or seep into the tanks and thereby contaminate the process. Such contamination may result in defective microelectronic parts and therefore can be quite costly.